EXPERIMENT 25PAGE 275

DR. SCOTT BUZBY PH.D.

Determination of the Dissociation Constant of a

Weak Acid

Objectives

Become more familiar with the operation of a pH meter

Quantitatively determine the equilibrium constant of a weak acid (Ka)

Acid-Base Equilibria (Strong Acids)

According to Brønsted-Lowry acid-base theory the strength of an acid is related to its ability to donate protons

A strong acid, HCl, will completely dissociate in dilute aqueous solutions

So the [H+] concentration of 0.1M HCl is 0.1M

)()(3)(2)( aqaqlaq ClOHOHHCl

Acid-Base Equilibria (Weak Acids)

By contrast, weak acids like acetic acid, HC2H3O2 (abbreviated HOAc or HAc) will only partially dissociate

The acid dissociation constant, Ka (equilibrium constant) is used to determine the concentration of H+:

)()(3)(2)( aqaqlaq AcOHOHHAc

][

]][[ 3

HAc

AcOHKa

Acid-Base Titration Curve

Equivalence Point

½ Equivalence Point

Equivalence Point

At equivalence point the moles of base added equals the moles of acid in unknown sample:

A

BBA V

VMM

AMpH log

½ Equivalence Point

½ equivalence point is the point where ½ the volume of base added to reach equivalence point has been added

At the ½ equivalence point pH of the solution is equal to the pKa of the unknown acid

ApH

A

A

A

K

KpH

KpH

pKpH

10

log

log

Experimental Procedure

Skip Part A – Page 279

Start with Part B – Page 280

Complete Parts C & D – Page 281

Due Next Week

Report Sheet – Pages 283-285

Questions – Pages 285-286

3 Graphs – Pages 287-289 (or Printouts)

Pre-Lab Experiment 27 – Page 313